I. Field of the Invention
The present invention relates to a process for producing a cadmium telluride polycrystalline film having grain sizes greater than about 20 .mu.m by first depositing cadmium telluride on a substrate and subsequently exposing the deposited cadmium telluride to an inert gas such as helium at an elevated temperature.
II. Description of the Prior Art
Polycrystalline, thin-film cadmium telluride shows promise for terrestrial photovoltaic applications primarily because of its direct band gap of 1.5 eV, which is optimum for solar energy conversion at air mass 1 (AM1). However, in order to fabricate high-efficiency solar energy collecting devices employing a cadmium telluride film, grain size of the cadmium telluride should be at least greater than 1 .mu.m, with increased efficiency being directly proportional to increased grain size. In particular, greater grain size avoids bulk recombination, large interface state density and high resistivity.
While cadmium sulfide films have been developed with grain sizes up to about 800 .mu.m, cadmium telluride films generally have not exceeded a grain size larger than about 5 .mu.m, with such growth occurring through post-deposition chemical treatment with cadmium chloride. Such small grain sizes and the resultant numerous boundaries of individual crystals critically inhibits the flow of charge carriers through a polycrystalline film.
In view of the potential value in photovoltaic applications for cadmium telluride films having large grain sizes, it is apparent that a need is present for a process for producing large-grain films. Accordingly, a primary object of the present invention is to provide a process for producing a cadmium telluride polycrystalline film having grain sizes greater than about 20 .mu.m.
Another object of the present invention is to provide such a process wherein cadmium telluride is deposited on a substrate by sublimation from a cadmium telluride effusion cell.
Yet another object of the present invention is to provide such a process wherein exposure to an inert gas is employed to enhance grain size development subsequent to cadmium telluride deposition on a substrate.
These and other objects of the present invention will become apparent throughout the description of the invention which now follows.